The basic function of a prosthetic heart valve is to provide a unidirectional blood flow valve in a mammalian (especially human) cardiac system to achieve efficient single direction blood circulation in the circulatory system when the valve is surgically implanted to replace a damaged or defective heart valve.
Typically, a prosthetic heart valve assembly, particularly one intended for human use, comprises a main valve body that comprises an annular ring member which is generally provided with skirt means to aid in surgically securing the ring member to a patient's cardiac system in a desired position. The ring member includes a generally centrally located passageway for flow therethrough of blood, and the passageway is provided with valve flow controlling means to achieve unidirectional flow of blood through the passageway. A valve flow controlling means can utilize a moveable ball or flap (in a form of a disc or leaflet) and includes guidance means that holds same in operative association with the ring member and that also allows same to move between open and closed positions. Examples of guidance means include a cage, projections from the ring member, recesses on the inner wall of the ring member with matching projections provided on the flow controlling means, and the like.
The efficacy of a heart valve prosthesis depends mainly on the blood flow regulating means, including its profile and its guidance means. In a particular design of a heart valve prosthesis, the construction materials used for its structure, design and fabrication, and its profile and localized wear characteristics, contribute to the prosthesis service capability and long-term operational reliability. Heart valve prosthesis failure can occur as a result of stagnation of blood leading to undesirable clot formation, tissue growth, excessive leakage of blood through or around the prosthesis, or other malfunction or disruption.
In a ball and cage type of heart valve prosthesis, the ball is made of synthetic polymeric material or of metal and is retained in a cage during its movements from open to closed positions (inclusive) to control unidirectional flow of the blood through the ring member which is made of metal. The disadvantages of such a prosthesis are mainly that the cage is fixed on the ring member and projects out from the ring member excessively into the heart chamber or the main artery, while the ball totally prevents desired central blood flow through the ring member when the valve is in its open position.
In a single disc (or single leaflet) prior art type of heart valve prosthesis, the disc is made either of material coated with pyrolytic carbon or synthetic polymeric materials, and the disc is mounted to move from open to closed positions (inclusive) either by projecting struts (or ears) or within a cage fixed on the ring member and usually made of metal. A disc having a flat or a curved surface does create some resistance to central flow of blood through the ring member when the disc is in its open position. As all portions of the struts or cage of a single disc prosthetic heart valve, as well as all portions of the ball and cage type prosthetic heart valve, are not wiped off equally or regularly, localized blood clots tend to form resulting in growth of tissues on them in due course. These tissues prevent free movement of the disc or the ball and prevent smooth flow of blood through the ring member particularly at later stages. These tissues also occasionally detach and produce embolisms. If wires associated with the cage or the pivoting means are welded to the ring member, there is potential danger of weld fracture and component separation at the welded joints.
Bi-leaflet (or double leaflet) prior art types of heart valve prostheses each have a ring member and two semi-circularly shaped leaflets to control unidirectional flow of blood through the ring member. The ring member and both the leaflets are preferably made of a material coated with pyrolytic carbon. Each leaflet has an outer peripheral circular edge that joins radial straight edge portions to form a semi-circularly shaped body. Each leaflet has a pair of opposed ears situated at locations on its circular edge adjacent to the ends of the straight edge portions. The ears are adapted to engage recesses provided on the inner wall of the ring member to form a hinge-like bearing structure. The two leaflets oscillate independently about their respective hinge axes and each leaflet in its closed position covers about one half of the passageway through the ring member. The two hinged leaflets as conventionally mounted across the passageway create some resistance to central flow of blood through the passageway when the leaflets are in their open positions. The ring member is unitarily constructed to incorporate the recesses for the ears.
An assembly process for such a prior art heart valve prothesis involves engaging the leaflet ears with the ring member recesses, and that assembly process must be accomplished by deforming the pyrolytic carbon coated ring member. A coating of pyrolytic carbon material is rigid or less flexible than the underlying material so that the engagement of the ears into the recesses must involve a minimum of flexing and a bare minimum bearing surface of engagement after flexing. A little more flexing is achieved either by providing two arm projections (preferably flat) for the recesses on either the inlet or the outlet side of the ring member, or, alternatively, by making the ring member thin in its cross-section so as to make it slightly more flexible. However, the possibilities of residual permanent deformation of the ring member, or of cracking of the ring member, in the assembly process cannot be completely eliminated, and the resulting minimum bearing engagement makes the ears of each leaflet prone to slipping or dislodgement from their recesses when the resulting prosthesis is in use.
The flat arm projections on the ring member make it very cumbersome and require extra efforts to insert the resulting heart valve prosthesis into an appropriate position in the patient's heart body, particularly during an aortic valve replacement operation. The alternative of a thin cross-section for the ring member hardly provides the minimum required rigidity for a ring member making the resulting heart valve prosthesis intrinsically a weak device. The unitary construction of the ring member with its recesses to accommodate the ears of the leaflets results in an inherent inability to adjust axial end-play precisely in the hinging mechanism such as is necessary to minimize malfunction, disruption and ultimate dislodgement of the leaflets over a period of time in use.
Thus, provision of either flat arm projections on the ring member, or a thin cross-section in the ring member, together with the minimum bearing surface engagement between the ears and the recesses, plus the inability to adjust axial end-play in the hinging mechanism, limits long-term reliable service of the presently available bi-leaflet heart valve prostheses.
Since safety of human life is involved, to achieve efficient and reliable service of heart valve prostheses, improved structures and fabrication processes are needed. Even a very small failure rate for a heart valve prosthesis is undesirable. There is a need for a new and improved heart valve prosthesis of the leaflet type which surpasses the performance and the reliability of presently available prior art heart valve prostheses. Moreover, an improved heart valve prosthesis should achieve optimal expected long service life characteristics. The present invention provides such an improved type of heart valve prosthesis.